On Mon, 24 Aug 2020, Alex Shi wrote: > Currently, compaction would get the lru_lock and then do page isolation > which works fine with pgdat->lru_lock, since any page isoltion would > compete for the lru_lock. If we want to change to memcg lru_lock, we > have to isolate the page before getting lru_lock, thus isoltion would > block page's memcg change which relay on page isoltion too. Then we > could safely use per memcg lru_lock later. > > The new page isolation use previous introduced TestClearPageLRU() + > pgdat lru locking which will be changed to memcg lru lock later. > > Hugh Dickins <[email protected]> fixed following bugs in this patch's > early version: > > Fix lots of crashes under compaction load: isolate_migratepages_block() > must clean up appropriately when rejecting a page, setting PageLRU again > if it had been cleared; and a put_page() after get_page_unless_zero() > cannot safely be done while holding locked_lruvec - it may turn out to > be the final put_page(), which will take an lruvec lock when PageLRU. > And move __isolate_lru_page_prepare back after get_page_unless_zero to > make trylock_page() safe: > trylock_page() is not safe to use at this time: its setting PG_locked > can race with the page being freed or allocated ("Bad page"), and can > also erase flags being set by one of those "sole owners" of a freshly > allocated page who use non-atomic __SetPageFlag(). > > Suggested-by: Johannes Weiner <[email protected]> > Signed-off-by: Hugh Dickins <[email protected]> > Signed-off-by: Alex Shi <[email protected]>
Okay, whatever. I was about to say Acked-by: Hugh Dickins <[email protected]> With my signed-off-by there, someone will ask if it should say "From: Hugh ..." at the top: no, it should not, this is Alex's patch, but I proposed some fixes to it, as you already acknowledged. A couple of comments below on the mm/vmscan.c part of it. > Cc: Andrew Morton <[email protected]> > Cc: Matthew Wilcox <[email protected]> > Cc: [email protected] > Cc: [email protected] > --- > include/linux/swap.h | 2 +- > mm/compaction.c | 42 +++++++++++++++++++++++++++++++++--------- > mm/vmscan.c | 46 ++++++++++++++++++++++++++-------------------- > 3 files changed, 60 insertions(+), 30 deletions(-) > > diff --git a/include/linux/swap.h b/include/linux/swap.h > index 43e6b3458f58..550fdfdc3506 100644 > --- a/include/linux/swap.h > +++ b/include/linux/swap.h > @@ -357,7 +357,7 @@ extern void lru_cache_add_inactive_or_unevictable(struct > page *page, > extern unsigned long zone_reclaimable_pages(struct zone *zone); > extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order, > gfp_t gfp_mask, nodemask_t *mask); > -extern int __isolate_lru_page(struct page *page, isolate_mode_t mode); > +extern int __isolate_lru_page_prepare(struct page *page, isolate_mode_t > mode); > extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg, > unsigned long nr_pages, > gfp_t gfp_mask, > diff --git a/mm/compaction.c b/mm/compaction.c > index 4e2c66869041..253382d99969 100644 > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -887,6 +887,7 @@ static bool too_many_isolated(pg_data_t *pgdat) > if (!valid_page && IS_ALIGNED(low_pfn, pageblock_nr_pages)) { > if (!cc->ignore_skip_hint && get_pageblock_skip(page)) { > low_pfn = end_pfn; > + page = NULL; > goto isolate_abort; > } > valid_page = page; > @@ -968,6 +969,21 @@ static bool too_many_isolated(pg_data_t *pgdat) > if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page)) > goto isolate_fail; > > + /* > + * Be careful not to clear PageLRU until after we're > + * sure the page is not being freed elsewhere -- the > + * page release code relies on it. > + */ > + if (unlikely(!get_page_unless_zero(page))) > + goto isolate_fail; > + > + if (__isolate_lru_page_prepare(page, isolate_mode) != 0) > + goto isolate_fail_put; > + > + /* Try isolate the page */ > + if (!TestClearPageLRU(page)) > + goto isolate_fail_put; > + > /* If we already hold the lock, we can skip some rechecking */ > if (!locked) { > locked = compact_lock_irqsave(&pgdat->lru_lock, > @@ -980,10 +996,6 @@ static bool too_many_isolated(pg_data_t *pgdat) > goto isolate_abort; > } > > - /* Recheck PageLRU and PageCompound under lock */ > - if (!PageLRU(page)) > - goto isolate_fail; > - > /* > * Page become compound since the non-locked check, > * and it's on LRU. It can only be a THP so the order > @@ -991,16 +1003,13 @@ static bool too_many_isolated(pg_data_t *pgdat) > */ > if (unlikely(PageCompound(page) && !cc->alloc_contig)) { > low_pfn += compound_nr(page) - 1; > - goto isolate_fail; > + SetPageLRU(page); > + goto isolate_fail_put; > } > } > > lruvec = mem_cgroup_page_lruvec(page, pgdat); > > - /* Try isolate the page */ > - if (__isolate_lru_page(page, isolate_mode) != 0) > - goto isolate_fail; > - > /* The whole page is taken off the LRU; skip the tail pages. */ > if (PageCompound(page)) > low_pfn += compound_nr(page) - 1; > @@ -1029,6 +1038,15 @@ static bool too_many_isolated(pg_data_t *pgdat) > } > > continue; > + > +isolate_fail_put: > + /* Avoid potential deadlock in freeing page under lru_lock */ > + if (locked) { > + spin_unlock_irqrestore(&pgdat->lru_lock, flags); > + locked = false; > + } > + put_page(page); > + > isolate_fail: > if (!skip_on_failure) > continue; > @@ -1065,9 +1083,15 @@ static bool too_many_isolated(pg_data_t *pgdat) > if (unlikely(low_pfn > end_pfn)) > low_pfn = end_pfn; > > + page = NULL; > + > isolate_abort: > if (locked) > spin_unlock_irqrestore(&pgdat->lru_lock, flags); > + if (page) { > + SetPageLRU(page); > + put_page(page); > + } > > /* > * Updated the cached scanner pfn once the pageblock has been scanned > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 1b3e0eeaad64..48b50695f883 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1538,20 +1538,20 @@ unsigned int reclaim_clean_pages_from_list(struct > zone *zone, > * > * returns 0 on success, -ve errno on failure. > */ > -int __isolate_lru_page(struct page *page, isolate_mode_t mode) > +int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode) > { > int ret = -EINVAL; > > - /* Only take pages on the LRU. */ > - if (!PageLRU(page)) > - return ret; > - > /* Compaction should not handle unevictable pages but CMA can do so */ > if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE)) > return ret; > > ret = -EBUSY; > > + /* Only take pages on the LRU. */ > + if (!PageLRU(page)) > + return ret; > + So here you do deal with that BUG() issue. But I'd prefer you to leave it as I suggested in 16/32, just start with "int ret = -EBUSY;" and don't rearrange the checks here at all. I say that partly because the !PageLRU check is very important (when called for compaction), and the easier it is to find (at the very start), the less anxious I get! > /* > * To minimise LRU disruption, the caller can indicate that it only > * wants to isolate pages it will be able to operate on without > @@ -1592,20 +1592,9 @@ int __isolate_lru_page(struct page *page, > isolate_mode_t mode) > if ((mode & ISOLATE_UNMAPPED) && page_mapped(page)) > return ret; > > - if (likely(get_page_unless_zero(page))) { > - /* > - * Be careful not to clear PageLRU until after we're > - * sure the page is not being freed elsewhere -- the > - * page release code relies on it. > - */ > - ClearPageLRU(page); > - ret = 0; > - } > - > - return ret; > + return 0; > } > > - > /* > * Update LRU sizes after isolating pages. The LRU size updates must > * be complete before mem_cgroup_update_lru_size due to a sanity check. > @@ -1685,17 +1674,34 @@ static unsigned long isolate_lru_pages(unsigned long > nr_to_scan, > * only when the page is being freed somewhere else. > */ > scan += nr_pages; > - switch (__isolate_lru_page(page, mode)) { > + switch (__isolate_lru_page_prepare(page, mode)) { > case 0: > + /* > + * Be careful not to clear PageLRU until after we're > + * sure the page is not being freed elsewhere -- the > + * page release code relies on it. > + */ > + if (unlikely(!get_page_unless_zero(page))) > + goto busy; > + > + if (!TestClearPageLRU(page)) { > + /* > + * This page may in other isolation path, > + * but we still hold lru_lock. > + */ > + put_page(page); > + goto busy; > + } > + > nr_taken += nr_pages; > nr_zone_taken[page_zonenum(page)] += nr_pages; > list_move(&page->lru, dst); > break; > - > +busy: > case -EBUSY: It's a long time since I read a C manual. I had to try that out in a little test program: and it does seem to do the right thing. Maybe I'm just very ignorant, and everybody else finds that natural: but I'd feel more comfortable with the busy label on the line after the "case -EBUSY:" - wouldn't you? You could, of course, change that "case -EBUSY" to "default", and delete the "default: BUG();" that follows: whatever you prefer. > /* else it is being freed elsewhere */ > list_move(&page->lru, src); > - continue; > + break; Aha. Yes, I like that change, I'm not going to throw a tantrum, accusing you of sneaking in unrelated changes etc. You made me look back at the history: it was "continue" from back in the days of lumpy reclaim, when there was stuff after the switch statement which needed to be skipped in the -EBUSY case. "break" looks more natural to me now. > > default: > BUG(); > -- > 1.8.3.1
